At present, a vectoring crosstalk cancellation (Vectoring) system includes a central office end and a far end. The central office end includes multiple (M) transceivers, and the far end includes multiple (M) transceivers. During downlink transmission, the M transceivers of the central office end serve as transmit ends, the M transceivers of the far end serve as receive ends, and each transmit end is corresponding to one receive end.
Generally, power or a power spectral density (PSD for short) of a signal sent by a transmit end is under strict control. For example, total power of transmit signals must not exceed a limit of a specified maximum value. For another example, power (that is, power spectral density, PSD) of a transmit signal on each subcarrier is controlled by a PSD profile.
Due to a requirement for power control, it is required that a precoder located at a transmit end does not increase transmit power. In the prior art, a normalization factor λ is used to perform normalized power control on a precoding matrix P used by a precoder to obtain that P=λ·P, where λ is a number less than or equal to 1, and P is a precoding matrix after power control is performed. Because λ is a number less than or equal to 1, a sum of squared elements of each row in the precoding matrix P after power control is performed may be less than or equal to 1, and therefore, the precoding matrix P after power control is performed does not increase transmit power, so that a transmit signal, passing through the precoder, of each line can meet a requirement for control of transmit power.
After a precoder uses a normalization factor λ, which is equivalent to that all transmit signals are multiplied by λ, a signal received by a receive end is distorted, and therefore, a receive end needs to use a recovery factor 1/λ to recover a received signal, that is, a frequency domain equalizer (FEQ for short) matrix of an FEQ needs to be multiplied by the recovery factor 1/λ.
To recover a transmit signal at a receive end, in addition to multiplying a precoding matrix P by a normalization factor λ, an FEQ matrix must be multiplied by a recovery factor 1/λ; otherwise, when the normalization factor λ is applied in the precoder and the recovery factor 1/λ is not applied in the FEQ matrix, a receive signal is distorted. However, the precoding matrix P is multiplied by the normalization factor λ at a transmit end, the FEQ matrix is multiplied by the recovery factor 1/λ at a receive end, and it is relatively complex to strictly control application of the normalization factor λ in the precoder and application of the recovery factor 1/λ in the FEQ to occur at the same time, thereby increasing complexity of power control.
Further, in the foregoing power control method, all elements in the precoding matrix P are multiplied by the normalization factor λ, which is equivalent to that transmit signals in all lines are multiplied by the normalization factor λ for reduction and weakening. When crosstalk signals of only a few lines are very strong (that is, only some elements in the precoding matrix P are very large), the foregoing power control weakens transmit signals of other lines whose crosstalk signals are not strong, which reduces signal transmission performance of an entire line.
Therefore, the power control method in the prior art has problems that control is complex and signal transmission performance of an entire line is reduced.